The invention relates to an X-ray image intensifier tube, comprising an entrance screen, an exit screen and electron-optical means for projecting photo-electrons from the entrance screen onto the exit screen, the entrance screen successively comprising a luminescent layer, an intermediate layer and a photocathode.
An X-ray image intensifier tube of this kind is known from U.S. Pat. No. 3,838,273. For a variety of reasons a barrier layer may be provided between the luminescent layer and the photocathode in X-ray image intensifier tubes. An example in this respect is a layer for chemical separation in order to preclude detrimental interaction between the layers; in particular contamination of the photocathode by substances released by the luminescent material, notably during the formation of the photocathode, is to be avoided. An intermediate layer of this kind is described in U.S. Pat. No. 3,706,885.
There may also be provided an intermediate layer which specifically serves to improve the electrical conductivity across the layer in order to prevent the occurrence of charging phenomena causing disturbances in the image. Such a layer is also described in U.S. Pat. No. 3,706,885. A layer for optical adaptation upon transfer of luminescent light from the luminescent layer to the photocathode is described in EP 199 426. An absorbing layer for locally influencing the transfer of luminescent light to the photocathode in a different manner, for example an anti-vignetting layer, is discribed in DE 21.34.110.
Various of these functions can be combined, either by using a multiple layer or by choosing a material such that the layer combines the function of, for examle chemical barrier layer with an improved electrical conductivity and/or optical adaptation. An example in this respect is described in EP 265 997.
The electron-optical system in X-ray image intesifier tubes aims to realize a uniform field strenght on the photocathode surface in order to form a well-defined image of the photo-electrons emanating from the photocathode on the exit screen.
However, regardless of the presence of absence of an intermediate layer, a disturbance which reduces the resolution of the tube always occurs in known tubes, even when the operation of the electron-optical system is optimum.